The parts illustration used by Draganfly looks like it is from the 1949-53 parts book.
That book does not give all the information for the earlier models.
I have checked the 1949 and 1950 parts books, which were issued at the time and which covered all models in the range.
The gearbox page notes that the oil seal, part 65 3508, was fitted on and after frame number ZB31 7840 for rigid frame and on and after ZB31S 6032 for plunger frame, which puts the change sometime during the 1949 or 1950 season, boxes without the oil seal would have sprockets with an oil scroll.